error LNK2019: unresolved external symbol "public: bool __thiscall

Hi all, I am working on a videogame being developed in C++. We use Ogre as our graphics engine.

I get the following error:

1>HostAPI.obj : error LNK2019: unresolved external symbol "public: bool __thiscall Star::ContainerManager::objectsInZone(int,bool,class Star::Container &,class std::vector<int,class std::allocator<int> > &)

Here is the portion of code in class HostAPI.cpp where the error occurs:

int allSuppliesInShelter(lua_State* pLuaState)
{
         int zoneID = static_cast<int>(lua_tonumber(pLuaState,1));
         Container& parentNode = *(Star::ContainerManager::getSingleton().getRoot());
         std::vector<int> objectTypes;
         objectTypes.push_back(71);

         bool allSuppliesInShelter = Star::ContainerManager::getSingleton().objectsInZone(zoneID, true, parentNode, objectTypes);

         lua_pushboolean(pLuaState, true);

         return 1;
} // End allSuppliesInShelter

The function objectsInZone is defined as Public in class ContainerManager as follows:

bool objectsInZone(int zoneID, bool allObjects, Container& parentNode, std::vector<int>& objectTypes)
{
        ContainerMap::iterator contentsIterator;

        for (contentsIterator = parentNode.getContents()->begin(); contentsIterator != parentNode.getContents()->end(); contentsIterator++)
        {
                    Star::Logger::getSingletonPtr()->logf(Star::Logger::LOG_TRIVIAL, "The objectType of the object is %d", (contentsIterator->second)->getObjectType());

                    if ((contentsIterator->second)->getObjectType() == 71)
        {
              //if (allObjects)
              //{
            // In case all objects of the types specified have to be in the zone.
                        
            //} // End if (allObjects)
      } // End if ((contentsIterator->second)->getObjectType() == 71)
      } // End for

      if (allObjects)
      {
             return true;
       } // End if
       else
       {
              return false;
        } // End else
} // End objectsInZone

Class ContainerManager is included in HostAPI.cpp:

#include <ContainerManager/ContainerManager.h>

Does anybody know why this error could be occuring?